The invention herein described relates generally to axial piston pumps and, more particularly, to an internally supercharged axial piston pump.
An axial piston pump has a barrel rotatably mounted within a pump housing. The barrel includes a plurality of circumferentially equally spaced bores in which pistons reciprocate. Each piston bore has a port in the end of the barrel that lies against a port plate that contains delivery and exhaust ports. As the barrel rotates, each piston bore port sequentially traverses the delivery and exhaust ports. As each piston bore port traverses the delivery port low pressure fluid is drawn into the piston bore. When the piston bore port traverses the exhaust port, fluid is expelled at an increased pressure.
The speed at which an axial piston pump may be run is limited by the rate at which fluid at the delivery port fills the piston bores during the pumping operation. If the piston bores are not filled with fluid as they traverse the delivery port, cavitation occurs, power is lost and severe damage to the pump may occur. Heretofore, booster pumps have been used to pressurize the fluid at the pump inlet in order to increase the filling speed of the piston bores and thereby increase the speed at which the pump may be operated. Booster pumps, however, add to cost and also occupy space which may be at a premium. Furthermore, booster pumps are commonly operated to increase the fill rate of the incoming fluid to a level sufficient to fill the barrel bores at the maximum operating speed of the pump. However, since a pump is not always operated at its maximum speed, the booster pump is providing supercharged fluid at a greater pressure than is necessary for a portion of the time the pump is operating, which results in wasted energy.
The present invention provides an axial piston pump that enables fluid entering the pump to be pre-charged without the addition of an auxiliary pumping mechanism or other type of external fluid precharge. The axial piston pump comprises a housing having a cylindrical inner wall surface surrounding a barrel chamber, a barrel mounted for rotation within the barrel chamber in the housing and having a plurality of circumferentially spaced piston bores therein, and a plurality of pistons reciprocally movable in the piston bores for pumping fluid from a delivery passage to an exhaust passage. In accordance with the invention, the barrel has at least one and preferably plural impeller vanes projecting radially outwardly and terminating at a radially outer vane edge adjacent the inner wall surface of the barrel chamber. Upon rotation of the barrel, the impeller vanes function to supercharge the fluid supplied to the piston bores.
In a preferred embodiment, the piston barrel comprises a core including the piston bores, and a sleeve surrounding the core, the sleeve including a cylindrical hub portion, and the impeller blade or blades projecting radially outwardly from the hub portion. The hub portion and the impeller blade or blades preferably are formed as a unitary piece, as by molding from plastic.
More particularly, the present invention provides an axial piston fluid pump comprising a housing having an inner wall surface surrounding a barrel chamber and a port surface at a first end of the barrel chamber, the port surface including a delivery port and an exhaust port circumferentially spaced apart in relation to a center axis of the barrel chamber; a barrel rotatably mounted within the barrel chamber in the housing and having a plurality of axially extending; circumferentially spaced piston bores therein, each piston bore having associated therewith a cylinder port in an end wall of the barrel located adjacent the port surface which cylinder port sequentially communicates with the delivery and exhaust ports during rotation of the barrel in the barrel chamber; a plurality of pistons disposed in the piston bores for reciprocation; and a drive shaft for rotatably driving the barrel in the barrel chamber. The housing further includes an inlet passage for delivering low pressure fluid to a second end of the barrel chamber opposite the port surface. In accordance with the invention, the barrel has a radially outer surface radially inwardly spaced from the inner wall surface of the barrel chamber to form an impeller pump chamber, and at least one and preferably a plurality of impeller vanes project radially outwardly from the outer wall surface of the barrel and terminate at a radially outer vane edge adjacent the inner wall surface of the barrel chamber. The impeller pump chamber has an inlet end in fluid communication with the second end of the barrel chamber and an outlet end in fluid communication with the delivery port, whereby upon rotation of the barrel in the barrel chamber, low pressure fluid from the second end of the barrel chamber is supercharged by the impeller vane prior to passage through the delivery port.
In a preferred embodiment, the drive shaft passes through the center of the barrel. The barrel may be axially slidable on the shaft and axially biased against the port surface. The drive shaft may be rotatably supported in the housing by bearings at opposite ends of the housing, which bearings carry the hydraulic loading acting on the barrel as is preferred.
In a preferred embodiment, the impeller vanes are circumferentially equally spaced around the barrel. Each vane preferably has a helical portion and an axial portion, and none of the vanes axially overlap an adjacent vane, as is desirable to facilitate molding of the vanes. According to another embodiment, each vane may be helical and of progressively increasing circumferential width going from the inlet to the outlet end of the impeller pump chamber, whereby the circumferential spacing between relatively adjacent vanes progressively decreases going from the inlet to the outlet end of the impeller pump chamber.
In a preferred embodiment, the port surface further has an annular discharge groove at the outlet end of the impeller pump chamber for receiving supercharged fluid and directing the supercharged fluid to the delivery port. The discharge groove preferably is connected to the delivery port by a volute, and the discharge groove preferably progressively increases in cross-sectional area in the direction of rotation of the barrel.
According to another aspect of the invention, a piston barrel for an axial piston pump comprises a core including a plurality of circumferentially spaced piston bores, and a sleeve surrounding the core, the sleeve including a cylindrical hub portion and at least one impeller blade projecting radially outwardly and termination at a radially outer vane edge.